This will address #69. Includes some pretty major changes but is currently broken; will force push and break up into smaller commits on some later date.

Edit: I think the best approach may be to entirely subclass/replace RcParams, monkey patch matplotlib.rcParams, and simply have proplot.rc point to matplotlib.rcParams. This will let me implement some other features, e.g. arbitrary units for matplotlib params with conversion upon calling RcParams.__getitem__, and keeps me from having to store separate dictionaries. The sections on the website docs will instead read "Shorthand params" and "rcParams additions"; the former will be implemented only in RcParams.__setitem__, and the latter will be literally added as valid matplotlib.rcParams. This will be yet another exercise in integrating proplot's internals more closely with the matplotlib API; wish I had looked at the matplotlib source code more in the early days of development.